186 
SIR J. F. W. HERSCHEL ON THE ACTION OF THE RAYS 
gible rays fell, and where the discoloration was in progress under their agency. For 
it was observed that, under these circumstances, the discoloration in question went 
on with much greater rapidity, so much so indeed, that the same amount of it, which 
without extraneous heat would have required twenty minutes or half an hour’s ex- 
posure to the spectrum to produce, was now produced in two or three minutes. Ob- 
scure terrestrial heat, therefore, is shown to be capable of assisting and being assisted 
in operating this peculiar change, by those rays of the spectrum, whether luminous 
or thermic, which occupy its red, yellow, and green regions ; while on the other hand 
it receives no such assistance from the purely thermic rays beyond the spectrum, 
acting under precisely similar circumstances, and in an equal state of condensation. 
161. When heat was similarly applied by radiation from behind, and from a non- 
luminous source, over the more refrangible region of a spectrum thrown on paper 
simply washed with tincture of guaiacum and not previously blued either by chlorine 
or by light, the blue colour induced in the more refrangible rays was still pro- 
duced, and of the same tint in the same points as if no heat had acted. This effect, 
the contrary to what the previous experiment would have led to expect, shows how 
little any reasonings on these points enable us at present to anticipate experience. 
162. The discharge of colour from blued guaiacum by mere heat, has been shown 
above (Art. 156.) to take place at a much lower temperature in the presence of 
moisture than when dry; and a similar destruction of colour, under similar circum- 
stances, takes place with many other vegetable preparations. Paper, for instance, co- 
loured with the juice of the Viola tricolor (Art. 90.), is speedily whitened in the dark, 
while wet, by the heat of boiling water, though dry heat does not affect it. And 
under the action of the spectrum it is discoloured (though much more slowly) by the 
same, or nearly the same rays which are effective in the case of guaiacum. The co- 
lour of paper tinged with the juice of the common red stock is not affected when 
dry by any heat short of what suffices to scorch the paper, but when wet (as when 
exposed to steam) it is speedily discharged. There are few, if any vegetable colours 
indeed which long resist the combined effects of heat and moisture, even when light 
is excluded, still less when admitted*. 
Of the Colours of Flowers in general under the action of the Spectrum. 
163. In operating on the colours of flowers I have usually proceeded as follows : — 
the petals of the fresh flowers, or rather such parts of them as possessed a uniform 
tint, were crushed to a pulp in a marble mortar, either alone, or with addition of 
alcohol, and the juice expressed by squeezing the pulp in a clean linen or cotton cloth. 
It was then spread on paper with a flat brush, and dried in the air without artificial 
* On the effects of light, air, and moisture at common temperatures, as discolouring agents on several dyeing 
materials, I may refer to M. Chevreul’s elaborate memoir (Acad. R. des Sciences, tom. xvi.). M. Chevreul’s 
experiments, however, relate to the action of light simply as it comes from the sun without prismatic separa- 
tion, and have therefore little or nothing in common with the objects of this paper. 
